5.3.4 The \tweak command

Changing grob properties
with \override causes the changes to apply to all of the
given grobs in the context at the moment the change applies.
Sometimes, however, it is desirable to have changes apply to just
one grob, rather than to all grobs in the affected context. This is
accomplished with the \tweak command, which has the following
syntax:

\tweak [layout-object.]grob-propertyvalue

Specifying layout-object is optional.
The \tweak command applies to the music object that immediately
follows value in the music stream.

For an introduction to the syntax and uses of the tweak command
see Tweaking methods.

When several similar items are placed at the same musical moment,
the \override command cannot be used to modify just one of
them – this is where the \tweak command must be used.
Items which may appear more than once at the same musical moment
include the following:

note heads of notes inside a chord

articulation signs on a single note

ties between notes in a chord

tuplet brackets starting at the same time

In this example, the color of one note head and the type of another
note head are modified within a single chord:

< c''
\tweak color #red
d''
g''
\tweak duration-log #1
a''
> 4

\tweak can be used to modify slurs:

\relative { c'-\tweak thickness #5 ( d e f) }

For the \tweak command to work, it must
remain immediately adjacent to the object to which it is
to apply after the input file has been converted to a music stream.
Tweaking a whole chord does not do anything since its music event
only acts as a container, and all layout objects are created from events
inside of the EventChord:

The simple \tweak command cannot be used to modify any object
that is not directly created from the input. In particular
it will not affect stems, automatic
beams or accidentals, since these are generated later by
NoteHead layout objects rather than by music elements in the
input stream.

Such indirectly created layout objects can be tweaked using the form
of the \tweak command in which the grob name is specified
explicitly:

\tweak cannot be used to modify clefs or time
signatures, since these become separated from any preceding
\tweak command in the input stream by the automatic
insertion of extra elements required to specify the context.

Several \tweak commands may be placed before a
notational element – all affect it:

The music stream which is generated from a section of an input file,
including any automatically inserted elements, may be examined,
see Displaying music expressions. This may be helpful in
determining what may be modified by a \tweak command, or
in determining how to adjust the input to make a \tweak
apply.